The invention relates to a process for manufacturing molded parts by polymerization of lactams in molds with addition of fiber composite material. The invention relates furthermore to novel molded parts produced according to the inventive process.
The technical manufacture of molded parts from lactams, preferably by activated alkaline quick polymerization or block polymerization has been in use for approximately 30 years. The term molded parts in the context of the present invention refers to elements that can be produced with the known casting and polymerization techniques and which include semi-finished products, shaped elements etc.
In the field of machine and apparatus construction, for example, it has been attempted for many years to replace sheet metal with other materials that have technical and/or financial advantages in their manufacture and/or their application. As alternative materials, among others, plastic materials have found acceptance which have versatile properties and can be matched very precisely to respective requirements. However, to date no plastic material has been found with which can replace metals completely with respect to strength, stability, hardness, stiffness, tenacity, temperature resistance, behavior in fire etc. Insofar as individual plastic materials have at least partially reached the technological properties of metal or in some cases even surpass them, their use is, in general, not comparable to the use of metals with respect to economic considerations. It is thus still necessary to select for each application the respective optimal replacement plastic material from a great palette of possible variants in order to comply optimally with the respective profile of specifications.
Fields for the use of sheet plastic molded parts are the automobile industry and transportation. In these fields sheet molded parts are used which in the past have been made from sheet metal. In the recent past, these molded parts have been replaced in particular by thermosetting materials, whereby especially unsaturated polyester resins (UP resins), polyurethanes (PU resins), and epoxide resins (EP resins) have been popular. For mechanically minimally loaded parts, thermoplastic materials have been used more and more whereby mostly the mass-produced plastics polypropylene (PP), polycarbonate (PC), acrylonitrile butadiene styrene (ABS), and different mixtures are being used.
The plurality of employed plastic materials, especially in the automobile industry, has been considered more and more as a disadvantage in comparison to the metallic materials because in the recent past a need for recycling and reuse of the discarded old apparatus has developed. While the metal recycling problem has been solved for years substantially by scrap metal treatment, plastic recycling is still in the start-up phase. It has been shown already that a sorting according to plastic types is very complicated and expensive. Instead, it seams to be more useful to employ from the beginning only a small number of plastic materials. These plastic materials should then be used for the respective applications in large quantities, in the automobile industry, for example, for bumpers, side panels, fenders, roofs, the underbody groups, engine compartment hood etc. In this context, it is desirable to provide only one plastic material for a large number of applications.
Important parameters for recycling of plastics is the required energy expenditure and the reworking degree of the remaining plastic to new plastic parts. Conventionally, the plastic parts are comminuted and foreign materials are cleaned off. The resulting raw material is, in general, available in granular form. When a thermoplastic material is provided it can be used completely in injection molding machines to produce new parts. However, thermosetting plastics do not melt under comparable conditions and can only be used partially as a filler material when new thermosetting parts are produced. Thus, the thermoplastic materials due to their complete recyclability have become more and more important in comparison to thermosetting materials so that more emphasis has been placed on the search for suitable thermoplastic replacement materials.
A known thermoplastic material with an especially balanced property spectrum is polyamide which is already being used for manufacturing high quality technical parts. Polyamides can be affected with respect to their technological properties so that they are interesting as a replacement for metals with respect to technical and economic considerations. However, thermoplastics could not be used in the past for producing sheet molded parts, i.e., parts that have thin walls relative to their surface area. Especially for supporting parts, which must withstand strong mechanical forces without breaking, thermosetting elements in addition to sheet metal still dominate.
Molded parts of polyamide can be produced, for example, by polymerization of lactams. The technical manufacture of molded parts from lactams, preferably by activated alkaline quick polymerization or block polymerization, has been used for approximately 30 years. The term sheet molded parts in the context of the present invention includes conventionally produced molded parts made of foil, sheet metal, or other large surface area materials which conventionally have a large surface area and a relative minimal thickness.
In general, molded parts of lactams are produced by polymerization of lactams in casting molds and, in comparison to conventional plastic elements, for example, also comprised of polyamide, are in general harder, stiffer, and more wear resistant. This is primarily a result of their higher crystallinity. By predetermining the method parameters such as temperature, residence time, etc., as well as by selecting the additives to be used and the post-treatment, the respective technological properties of the molded parts can be affected. Most of these aforementioned conditions, however, can only be maintained with very great economical and technical expenditure. The high crystallinity has primarily the effect that the shaped parts produced from lactams also have a high brittleness. Thus, the molded parts produced from lactams in conventional methods can be destroyed in an explosion-like fracture by suddenly occurring overloads due to a so-called cold flow. This can result in considerable damage in connection with constructive parts relevant to safety aspects.
Even though in the prior art the use of fiber glass-reinforced thermosetting resins cured from monomers in molds, these prior art methods cannot be simply transferred by a person skilled in the art. Thermosetting plastics, i.e., fiber glass plastics in the conventional sense, are produced by radical-induced polymerization. The manufacture of molded parts with polymerization of lactams relates to thermoplastic materials produced by anionic polymerization. This process concerns parameters that are of no consequence in radical polymerization. Thus, the transfer of methodological knowledge of the thermosetting plastic manufacture cannot be simply applied to the thermoplastic manufacture by lactam polymerization since the anionic polymerization is subject to its own rules. The rules for thermosetting plastic manufacture cannot be transferred.
Special problems for the anionic polymerization of lactams result, for example, from method parameters in regard to moisture and the sensitivity of the lactams to polar substances. For example, it is not possible to embed conventional fiber glass plastics into the lactams, but they must fulfill certain criteria in regard to the method parameters.
Methods for manufacturing molded parts by polymerization of lactams are, for example, known from DE 1 174 982 A1 and DE 1 910 175 A1. The first mentioned method discloses the manufacture of cast polyamide shaped bodies which are produced by rotational casting whereby the rotational forces may not surpass the gravitational forces. Even though the general possibility of introducing filler materials is disclosed, it is not recognized here with which measures defined technological properties can be produced. DE 1 910 175 A1 discloses also a method for producing polyamide cast parts whereby mats are introduced for increasing the stiffness of thin-walled molded parts. There is no disclosure in regard to the directed use of fibers or the use of a certain type of fibers.
In DE 2 050 572 A1, DE 1 214 865 A1, and DE 1 066 012 A1, rotational and spinning cast methods are disclosed which describe method details but no relation to the manufacture of molded parts fiber-reinforced in a directed manner.
It is an object of the present invention to improve a method for manufacturing molded parts by polymerization of lactams in molds such that the resistance of the inventively produced molded parts with respect to suddenly occurring load peaks is increased and the reproducibility of the defined technological properties of the inventively produced molded parts is improved.